A three-way GOP primary may be in store as gubernatorial candidate Tom Campbell considers switching races and running against multimillionaire Carly Fiorina and Assemblyman Chuck DeVore.

The race to unseat U.S. Sen. Barbara Boxer is growing increasingly volatile: The presumed Republican front-runner's fundraising is anemic, the underdog is attracting the party's most passionate voters, and a third major GOP politician is pondering whether to leap into the contest.

For months, multimillionaire businesswoman Carly Fiorina and conservative Orange County Assemblyman Chuck DeVore have brawled in a race that mirrors the philosophical debate cleaving the GOP nationwide -- a candidate favored by the party's pragmatic establishment facing a passionate challenge from one fancied by its conservative base.

But Fiorina's candidacy is being undermined by unexpectedly lackluster fundraising -- a weakness that undercuts one of her strongest rationales for securing the nomination. Further roiling matters is the potential entry of former U.S. Rep. Tom Campbell, who is a gubernatorial candidate. Campbell is widely believed to be switching races and is expected to announce his decision Thursday.

The conflict among Republicans threatens what many see as the GOP's best chance against the three-term Democratic senator since she won the seat in 1992. Anti-incumbent fervor is rippling across the nation against the party that controls the White House and Congress.

"This could be the toughest race Sen. Boxer has faced, with the recession, the off-year electorate, and with one of our opponents, who could spend millions of her personal fortune," said Rose Kapolczynski, Boxer's campaign manager.

Fiorina's wealth -- she made tens of millions of dollars in severance alone when she was fired as chief executive of Hewlett-Packard -- instantly made her a potent challenger even before she announced her candidacy in November. Her entrance had been delayed while she received treatment for breast cancer.

Despite having said repeatedly that she would not finance her effort, Fiorina announced Monday that she lent her campaign $2.5 million before the Dec. 31 financial filing deadline.

"I really wanted to get this campaign off and running to a very strong start," she said. "We declared quite late because of my earlier battle with cancer and I wanted to be sure we could hire the best and really ramp up."

Fiorina noted that she raised more than $1 million in November and December, despite the dormant period around the holidays. But critics point out that without her loan, Fiorina would have had only $158,000 after debts at the filing deadline. That would have put her only slightly ahead of DeVore.

Democrats were quick to contrast Fiorina's showing with that of fellow Republican and former EBay head Meg Whitman, who raised $6.5 million within five months of forming an exploratory committee for the gubernatorial race.

"The bottom line is Fiorina has few friends," said Bob Mulholland, campaign advisor to the state Democratic Party.

Fiorina is promoting herself as the only nonpolitician in the race, a problem-solver who rose from secretarial temp to become the only woman to head a Fortune 20 company. While she says her values match DeVore's, she also deems herself more electable, in part because she is a woman.

"With all due respect and deep affection for white men -- I am married to one -- but she knows how to beat them in California," Fiorina said of Boxer. "She's done it over and over and over."

DeVore has been reaching out to voters for more than a year to boost his reputation as the candidate of firmly conservative fiscal and social positions. DeVore's campaign is also pushing the claim that Fiorina is the candidate of the party elite.

The class warfare plays well with DeVore's base -- tea-party activists and others skeptical of party leaders in Sacramento and Washington. But it seems unlikely to turn into much financial support for DeVore from organized GOP groups, said Allan Hoffenblum, a Republican strategist. The cost to campaign in sprawling California is simply too great, he said.

All this has given Fiorina a slight edge, but Campbell's entrance into the race could upend things. A former member of Congress and the state Legislature from the Silicon Valley, he pulled out of a gubernatorial candidate forum scheduled for Thursday night in Los Angeles, said Jane Barnett, chairwoman of the Republican Party of Los Angeles County.

Campbell's spokesman Jamie Fisfis confirmed that the candidate has been urged by Republican backers to switch to the Senate race. Fisfis refused to discuss Campbell's plans.

"There is a lot of information swirling out there. We're just proceeding with our plan," Fisfis said.

The race would be Campbell's third reach for a Senate seat: In 1992, he lost the GOP primary to Bruce Herschensohn, and in 2000, he won the nomination but was crushed by Sen. Dianne Feinstein.

In the primary for the governor's race, where he has faced two wealthy competitors willing to put millions behind their campaigns, Campbell's fundraising has been dreary. A question ahead is whether he would do any better in the Senate race, which has a far lower maximum individual contribution level.

His impact is also uncertain. Some believe he and Fiorina would divide moderates, benefiting DeVore. Alternately, he and DeVore could split male voters, benefiting Fiorina.

"Predicting three-way primaries is the single stupidest thing a political pundit can do," said Dan Schnur, director of the Jesse M. Unruh Institute of Politics at USC and a former Republican political operative.